When the audio amplifier is powered on, this produces unwanted noise which is called a “pop” by those skilled in the art. This pop is considered to be an audible defect to the user of the device. It is unpleasant to hear, particularly when listening through headphones or earphones.
FIG. 1 shows the evolution over time of the output voltage Vout(t) of the audio amplifier at power-on, in the absence of an input signal to the amplifier. The pop arises from the generation of a voltage peak 1, which is called a glitch, and the presence of a non-zero voltage level 2 corresponding to the offset voltage of the amplifier.
This offset voltage results from an imbalance between the two static currents respectively established in each of the two branches of the differential pair that forms the input stage of the amplifier.
In practice, it is not uncommon to observe offset voltages of around ±10 mV. A “pop” corresponding to a transient variation of 1 mV in the output can be heard by a user.
The offset voltage also causes static power consumption, which corresponds to the product of the offset voltage and the load impedance. In the applications envisaged here, the device is generally battery-powered, and reducing the static power consumption is desirable.
Techniques exist for decreasing the unwanted noise in the audio signal reproduced for the user, arising from the offset voltage of the audio amplifier. In principle, the techniques previously considered here consist of observing the output voltage from the amplifier in the absence of an input signal, and controlling an offset cancellation means internal to the amplifier.
Such a technique is present in the article in the IEEE Journal Of Solid-State Circuits, Vol. 29, No. 5, May 1994, entitled “An Automatic offset compensation scheme with Ping-Pong control for CMOS operational amplifier”. However, the solution in this article does not apply to the audio applications envisaged, because of the lack of linearity in the offset cancellation that it provides.